Comminution and beneficiation of refractory flour



Jan. 12, 1965 w. H. OWEN ETAL 3,155,267

COMMINUTION AND BENEFICIATION OF REFRACTORY FLOUR Filed June 29, 1962CALCINED FEED CALCINED FEED +28 f cRusHER +16 O DRY PAN v E E v v 28MESH SCREEN SCREEN UNDERSlZE OTHER PRODUCTS V +48 |OO MESH SCREEN TPERMANENT -za +IOO IOO MESH MAGNETIC PULLEY MAGNETIC ELECTRO sEPARATORMAGNETIC -2 +|oo PULLEY FREE OF IRON PERMANENT MAGNETIC QE' PULLEYcERAMlc STORAGE HOPPER STORAGE HOPPER 2 F/ WiLLIAM H O E A ATTORNEYUnited States Patent ()fiiice Bdhbib? Patented Jan. 12, 1955 3,165,267COMMINUTIGN AND BENEFICIATION OF REFRACTORY FLQUR William H. Gwen andRobert K. Scott, Pittsburgh, Pa., assignors to Harbison-WalkerRefractories (Jompany, Pittsburgh, 1221., a corporation of PennsylvaniaFiled June 29, 1962, Ser. No. 206,373

' 8 Claims. (Cl. 241-24) This invention relates to comminution andbeneficiation of refractory flour, and more particularly, to a method ofproducing very high purity refractory flour having utility as aninvestment molding material.

. In precision casting of metal parts by the so-called lost wax process,it is conventional to use a very finely divided refractory flour and abinder solution to make a precoatslurry. For example, an expendable,fusible, waxlike pattern is made, the dimensions of which aresubstantially identical in all respects to a metal part which.

is to be cast. This pattern is sequentially dipped into the pre-coatslurry of refractory fiourand binder, the excess slurry drainedthereoif, and while still wet, subjected to a stuccoing or sanding witha coarse refractory grit. This sequence of steps is repeated the numberof times necessary to fabricate a shell or sheath of a desired thicknessabout the expendable pattern. The expendable pattern is then melted out,leaving the desired mold.

This ceramic mold is usually of very thin wall section, i.e. to A, isusually mounted in some manner of back-up such as sand or steel shoteither loosely consolidated or unconsolidated, and has molten metalpoured thereinto produce the precision casting desired. These molds areused for but one casting.

The surface finishand quality of precision castings made from thesemolds is affected if, in any of the precoat slurries used, there existsoccluded gases from such as air entrapped by poor mixing techniques,from gases generated by reactions between the refractory materials andthe binder solution, etc.

The incidence of reaction between the refractory materials and bindersolutions is considerably increased when the mold is rapidly heated to ahigh temperature during its contact with molten metal being cast. It isnot uncommon for this molten metal to be poured into the mold at atemperature of 2900 F. and even higher. The gases which are generatedunder, such high temperature conditions cause pinholing ofthe metalcasting. Furthermore, they sometimes cause reaction between the moltenmetal and the material of which the mold is made, in such a manner as tocause spaced areas of adhesion between the casting and the mold uponcooling. Such adhesion, during cooling, causes fissures or cracks tooccur in the cast shape, because of variances in the surface tension ofthe cooling metal. Irony materials are one of the pri mary causes ofsuch gaseous exudate and surface adhesion. Even in extremely smallamounts, i.e. fractions of 1%, the irony-materials are detrimental.

Accordingly, it is an object of this invention to provide an improvedmethod of comminution and beneficiation of refractory flour havingparticular utility as an investment molding media. It is another objectof this invention to provide investment molding media substantiallyentirely free of those impurities which would tend to react with bindersolutions or which would, under conditions of high temperature, producegas which would pin hole or otherwise damage a precision casting.

It is yet another object of this invention to provide a very finelydivided, refractory flour having less than about 0.001% metallic iron,on the basis of an oxide analysis. Audit is still another object of thisinvention to provide very'finely divided, iron-free mullite'flour havingpara ticular utility as an investment molding media for the so-called10st Wax and like processes.

Briefly, according to one embodiment of this invention, a technicalgrade mullite (calcined bauxite substan tially entirely of mullite phasewith minor amounts of corundum and glass) is provided, which hasessentially less than 0.001%, by weight, metallic iron on the basis ofan oxide analysis. This material is prepared by a process, whichincludes an initial crushing of a calcined feed to a mixture which issubstantially all 16 mesh Tyler. This crushed material is subjected toan initial screening step, which provides three fractions. The firstfraction is oversize or +16 mesh Tyler, which material is returned tothe initial crushing step. An intermediate fraction is 16-]-4 8 meshTyler, and is subjected to subsequent treatment steps. The 48 mesh Tylerfraction is discarded. I I

The intermediate l6+48 mesh fraction is subjected to magneticseparation, and thence to further comminution in a ceramic-linedballmill. The product of the ceramic mill is substantially all meshTyler, and is termed a flour in the art, which flour is substantiallyentirely free of irony materials.

A better understanding, other features, and further objects andadvantages of this invention will become obvious, to those skilled inthe art, from a study of the following'detailed description withreference to the drawings. In these drawings;

FIG. 1 is a schematic diagram of a preferred method of producing veryfinely divided beneficiated refractory flour, according to the conceptsof this invention;

FIG.'2 is a schematic diagram of an alternative method.

' The present invention is applicable to almost all refractory materialswhich are capable of being reduced to a flour for use in such asinvestment molding procedures. It is, thus, applicable to silica,alumina-silica, alumina, zircon, Zirconia, calcia, periclase,periclasesilica, chrome, chrome-periclase, etc., and like minerals. Forthe purposes of explanation, however, this invention will be describedin the manufacture of a very finely divided, beneficiated mullite flourfrom calcined bauxite (an alumina-silica type mineral). 1

As noted above, one of the primary purposes of the invention is toremove certain gas-forming agents, such as metallic iron and ironycompounds, which would react in service to produce undesired effects ona metal part being cast. In actual laboratory tests on refractoryinvestment materials, it was found that metallic iron, in amounts ofonly 50 parts .per million, was objectionable since it caused asufficient volume of gas to be given off in service as to damage. aprecision casting.

' Further, metallic iron reacts with water and other com- Thesereactionproducts often promote adherence of the investment molds to ferrouscastings causing fissures to appear in the castings upon cooling.

,7 The hydrogen -gas,' mentioned above, is a reaction product ofmetallic iron and water or other combined hydrogen, and coalesces to.form small bubbles within the investment mold. These coalesced bubblesappear 3 as small gas cavities within the'm'old which rupture uponheating to cause fissures through the working face of the mold intowhich molten metal will flow, resulting in a pimpled surface on theresulting casting. V

7 Although some of the metallic ironand irony mate rials found'in therefractories used for investment molding are naturally-occurring, mostof it appears to be picked up during grinding thereof to obtain thedesired very finely divided flour state. Most of this contamina,

tion appears in the extremely fine fractions of this flour,

and in such a range as to be incapable of separation by magnetictreatment.

Referring now to FIG. 1, according to' this invention a quantity ofbauxite havingthe following typical analysis (all partsby weight, and onthe basis ofan oxide analysis): I

. Percent Silica (SiO 15.5 Alumina (A1 0 54.6' Titania (TiO 1.9 Ironoxide (R 0 0.8 Calcium oxide (CaO) 0.1

Magnesia (MgO) Q Trace Alkalies (Na OA-K 'Oi-Li O) 0.1 Loss on ignitionT 27.2

was calcined to remove all free and substantially all chemicallycombined water, and to produce a calcined material having an ignitionloss on the order of up to about 0.1%. This calcining step maybe carriedout in such as a rotary kiln. an initial crushing step in a dry pan toproduce a material substantially all of which is -16 mesh Tylerv Ofcourse, if desired, a ball mill, rod mill, impact crushing mill, etc.,can be used for this step. From the dry pan,

the material passes to a screening step; The screening step may beconducted. in such asthe Rotex screen of the Orville-Simpson Company,which -is described on The calcined material is fedv to page 958 of theChemical Engineers Handbook, 3rd Edi- I tion. 1

Three fractions result from the screening step. The, first is a +16.mesh Tyler or oversize fraction, which is recycled to the dry pan. Thethird fraction is a -48 mesh Tyler or'und'ersize, fraction, which isdisposed of;

The intermediate or -16+48 mesh Tyler fraction is passed to a magnetictreatment to remove all of the iron and irony fluxes contained therein.

' the materialis sequentially subjected to a permanent magnet pulley ordrum-type separator, an electromagnetic pulley-type separator, and thento a second permanent magnet-type separator;

The sequence of permanent magnet-icand electromagnetic treatment stepsproduces a product having essen- "tially less than 0.001%, by weight,metallic iron and' irony materials. I This material is fed. toa ceramiclined mill of the ceramic ball mill type to produce a refractory flour.The product from the ceramic-ball mill is for subsequent packaging as,desired.

Previously, it has notbeen thought commercially feasible to produceinvestment molding flour by ceramic milling, because of the. greatexpense involvedin large ceramic mill installations. However, by ourpreliminary 1 separation and isolation of an intermediate pre-groundfraction, particularly amenable to magnetic separation I treatment, allof the material whichis passed through the ceramic mill is product andthere is no crushing of "material whichxmust be subsequently wasted.

In this treatment, 1

.fed to suitable;storage facilities, suchj'as a storagehopper,

2 is more difiicult to handle than the 48" mesh frac- I tion of themethod of FIG. 1. In any event, the frac= tion which passesto. themagnetic separatingstepimust be entirely mesh, and is preferably allless than .16 mesh. I

Two samplesof. mullite flour, which were actually produced according tothe method of FIG. 1, weresubjeeted to a very careful chemicalanalysisina laboratory. The first sample was substantially entirely mesh Tylerand the second was substantially entirely 200 V mesh Tyler. Each of thesamples was divided into two parts. A firstpart of each sample wastested qualita- *tively for the presence of iron or irony substances, by

wetting with water and adding concentrated hydrochloric acid. ThereWasno evidence of-gas evolution from either 251 of the samples, as wouldhave indicated the presence of iron. a

Quantitative chemical tests were then undertaken on the second portionof each of the samples. .The '150 mesh sample indicated 0.0002%, byweight, iron present. No evidence of iron'could be found in the 200 meshsample. Y

The following is a brief synopsis of the method used to quantitativelyanalyze the material for metallic iron:

A 100.0;gram sample of material, as. received, was transferred to a .250'r'nl. Erlenmeyer flask, and 501111.

of 4% HgCl solution was added. Using a trapto col lect. the condensateand a burner to ignite theeXit gas,

the airwas displaced from theflaskby a continuous flow of natural gas.Both the flask and sample were heated for 10 minutes, to just below;boilingl of the sample. ,The flask was then cooled in ice -Water andthe solution decanted through a 5 cm. #2 Whatman paper in a #1 Biichnerfunnel into a 250 ml; flaskcontaining 15 ml. of l-l H 80 The sample waswashed by decantation using 25-30 ml; of cold water for eachdecantation. Thefiltratewas transferred (225rnl.) to

. a 600 ml. beaker and titrated with astandard Ce(SO solution usingone'drop of o-phenanthroline-as anindicator. Itw'as then titrated to acolorless end-point. The sample was washed andthe' filtrate added to thetitrated solution until the indicatorfshowed no color" change, or 1 dropof Ce (S0 wassufiicientto discharge the color, toassure completeremoval'of ferrous salts from the saniple. 0.1 .ml. from thetitrationwas d ducted for the blank, and percentage .otiron was cal--culated. I r r I 7,

Calculation: I r

' .ml. O.2239

i Wt. sample' Nora-Preparation of Ce'(SO4 solution (0.05 8.4 grams ofCe(SO were dissolved in 'a solution containing l4 ml. of H 50 and 300ml.-water, It was 1 thenffiltered through a #5 Whatman paper using suc--tion, drluted t'o 500ml. and mixed .well. Treated as describedfiabove,it was standardized against 0.01001 to:

An alternative flow sheet, schematically shown in FIG. U

2 is useful for treating smaller quantities of ,material. It includesa-n-initial' calcining step, a' crushing step'to produceasubstantiallyr-ZS meshj', Tyler vmaterial, ja

first screening step to remove the oversize or, +28 mesh" 7 fraction,for'recycle to the crushen-a subsequent=l00 meshscreening step toremove a -l00'n1esh waste-prod;-

0.0200 gram of iron wire. p p

In an actual industrial testusing iron-free :mullite flour, producedaccording to the'method hof FIG; 1, very high quality precisioncastingswere obtained. In'addi- :-'tion to the precisionof the castings obtainedin the absence of metallic ironand irony;, substance s, :it was dis-vcovered that ain'old made of such a material resisted .wet-

ting bythe molteu'm'etali It is surmised that wetting, which had beennoted with previous molding fmaterials; 7

had been the result of deposits of irony material on the working surfaceof the mold, which encouraged wetting of the mold by the molten metal.However, with the iron-free material of this invention, wetting was notnoted.

Having thus described the invention in detail and with sufiicientparticularity as to enable those skilled in the art to practice it, whatwe desire to have protected by Letters Patent is set forth in thefollowing claims.

We claim:

1. Method of producing a refractory flour essentially free of ironymaterial for use as an investment molding media, which comprises thesteps of passing a selected refractory material to a preliminary sizereduction stage, isolating an intermediate fraction substantially all ofwhich rests on a 100 mesh Tyler screen from the reduced material,removing substantially all irony material from the intermediatefraction, passing the resulting material to a secondary size reductiontreatment in a ceramic mill, said secondary size reduction treatmentproducing a refractory flour substantially all of which is -150 meshTyler and substantially entirely free of irony materials.

2. Method of claim 1, in which the selected refractory material isbauxite.

3. Method of producing a refractory flour essentially free of ironymaterial for use as an investment molding media, which comprises thesteps of passing a selected refractory oxide material to a calciningstage, passing the calcined material to a preliminary size reductionstage, isolating an intermediate fraction substantially all of whichrests on a 100 mesh Tyler screen from the re duced material, removingsubstantially all irony material from the intermediate fraction, passingthe resulting material to a secondary size reduction treatment producinga refractory flour substantially all of which is 150 mesh Tyler andsubstantially entirely free of irony materials.

4. Method of claim 3, in which the refractory material is bauxite.

5. Method of producing a refractory flour essentially free of ironymaterial and having particular utility as an investment molding media,which comprises the steps of passing a selected refractory oxide to aninitial crushing step to produce a material which is substantially alll6 mesh Tyler, passing the crushed material to a separation stage, saidseparation stage dividing the crushed material into three fractions, thefirst fraction being +16 mesh Tyler oversize, returning said oversize tothe crushing step, a second fraction being l mesh Tyler and undersize,the third fraction being an intermediate l6-H00 mesh Tyler fraction,passing the intermediate fraction to a magnetic separation treatment, inwhich substantially all irony material is removed, passing the iron-freeintermediate fraction to a fine crushing stage in a ceramic mill tothereby produce a refractory flour substantially calcining a selectedrefractory oxide material to remove all free water and substantially allchemically combined water, passing the calcined material to a separationstage, said separation stage dividing the crushed material into threefractions, the first fraction being +16 mesh Tyler oversize, returningsaid oversize to the crushing step, a second fraction being mesh Tylerand undersize, the ttu'rd fraction being an intermediate 16-|-l00 meshTyler fraction, passing the intermediate fraction to a magneticseparation treatment, in which substantially all irony material isremoved, passing the iron-free intermediate fraction to a fine crushingstage in a ceramic mill to thereby produce a refractory floursubstantially free of irony materials.

7. Method of producing a refractory flour essentially free of ironymaterial for use as investment molding media, which comprises the stepsof passing a selected refractory material to a preliminary sizereduction stage, isolating an intermediate fraction substantially all ofwhich passes a 16 mesh screen and rests on a 100 mesh screen, treatingsaid material to reduce the irony content thereof to essentially lessthan 0.001%, passing the resulting material to a secondary sizereduction treatment in a ceramic mill, said secondary size reductiontreatment producing a refractory flour all of which is mesh Tyler andsubstantially entirely free of irony materials.

8. Method of producing a refractory flour essentially free of ironymaterial and having particular utility as an investment molding media,which comprises the steps of passing a selected refractory oxide to aninitial crushing step to produce a material which is substantially all-16 mesh Tyler, passing this crushed material to a separation stage, theseparation stage dividing the crushed material into three fractions, thefirst fraction being substantially +16 mesh Tyler oversize, returningsaid oversize to the crushing step, a second fraction being 100 meshTyler and undersize, discarding said undersize, the third frac tionbeing an intermediate 16+100 mesh Tyler fraction, passing thisintermediate fraction to a magnetic separation treatment in which theirony material content is reduced to less than 0.001%, passing thisiron-free intermediate fraction to a fine crushing stage in a ceramicmill to thereby produce a refractory flour substantially free of ironymaterials.

References Cited by the Examiner UNITED STATES PATENTS 1,898,371 2/33Hutchins 24124 X 2,269,912 1/42 Ladoo, et al. 241-24 2,468,321 4/49Bland 24124 2,603,423 7/52 Buehl 24l24 X 2,618,530 11/52 Gardner10638.27 2,840,480 6/58 Stoddard 106-3827 2,865,570 12/58 Nutting 241-24J. SPENCER OVERHOLSER, Primary Examiner.

MORRIS LIEBMAN, Examiner.

1. METHOD OF PRODUCING A REFRACTORY FLOUR ESSENTIALLY FREE OF IRONYMATERIAL FOR USE AS AN INVESTMENT MOLDING MEDIA, WHICH COMPRISES THESTEPS OF PASSING A SELECTED REFRACTORY MATERIAL TO A PRELIMINARY SIZEREDUCTION STAGE, ISOLATING AN INTERMEDIATE FRACTION SUBSTANTIALLY ALL OFWHICH RESTS ON A 100 MESH TYLER SCREEN FROM THE REDUCED MATERIAL,REMOVING SUBSTANTIALLY ALL IRONY MATERIAL FROM THE INTERMEDIATEFRACTION, PASSING THE RESULTING MATERIAL TO A SECONDARY SIZE REDUCTIONTREATMENT IN A CERAMIC MILL, SAID SECONDARY SIZE REDUCTION TREATMENTPRODUCING A REFRACTORY FLOUR SUBSTANTIALLY ALL OF WHICH IS -150 MESHTYLER AND SUBSTANTIALLY ENTIRELY FREE OF IRONY MATERIALS.